Containment system for constraining a prosthetic component

ABSTRACT

Methods, systems and devices for preventing prosthetic articulating surfaces from separating from each other. A containment system according to one embodiment seeks to prevent an implant stem head from dislocating from a prosthetic component while providing an increased range of motion over conventional contrained components. In one embodiment, an implant structural member includes a cavity and an opening having a lip, including a web along a portion of the lip. An implant stem head has a cooperating surface that corresponds with the web, so that when the cooperating surface of the implant stem head is aligned with the web, the head may be inserted into the implant structural member. The implant stem head is then rotated and the femoral stem component attached, thereby preventing dislocation of the head.

RELATED APPLICATIONS

This is a continuation of application Ser. No. 10/057,284, filed Jan.24, 2002, now abandoned, which claims priority to ProvisionalApplication No. 60/264,153, filed Jan. 25, 2001 entitled, “Captive Headfor Bipolar Endoprosthesis,” each of which is incorporated by referenceherein.

FIELD OF THE INVENTION

The invention is directed generally to methods, systems and devicesrelated to prosthetic implants, including a device for preventingprosthetic articulating surfaces from separating from each other, andmore specifically to a device for preventing an implant stem head fromdislocating from an a prosthetic component.

BACKGROUND OF THE INVENTION

Artificial implants, including hip joints, shoulder joints and kneejoints, are widely used in orthopedic surgery. Hip joint prostheses arecommon. The human hip joint acts mechanically as a ball and socketjoint, wherein the ball-shaped head of the femur is positioned withinthe socket-shaped acetabulum of the pelvis. Various degenerativediseases and injuries may require replacement of all or a portion of ahip using synthetic materials. Prosthetic components are generally madefrom either metals, ceramics, or plastics.

Total hip arthroplasty and hemi-arthroplasty are two procedures wellknown within the medical industry for replacing all or part of apatient's hip. A total hip arthroplasty replaces both the femoralcomponent and the acetabular surface of the joint, so that both afemoral prosthesis and an acetabular prosthesis are required. Aconventional acetabular prosthesis may include a cup, a cup and a liner,or in some cases only a liner, all of which may be formed in variousshapes and sizes. Generally, a metal cup and a polymeric liner are used.However, the liner may be made of a variety of materials, includingpolyethylene, ultra high molecular weight polyethylene and ceramicmaterials. The cup is usually of generally hemispherical shape andfeatures an outer, convex surface and an inner, concave surface that isadapted to receive a cup liner. The liner fits inside the cup and has aconvex and concave surface. The cup liner is the bearing element in theacetabular component assembly. The convex surface of the linercorresponds to the inner concave surface of the cup or acetabulum, andthe liner concave surface receives the head of a femoral component. Anacetabular cup may include a highly polished inner surface in order todecrease wear.

The liner concave surface, or internal concave surface, is characterizedby features relative to an axis through the center of the concavesurface. This axis may or may not be aligned with the central axis ofthe shell. In a typical liner the concave surface has a hemisphericalgeometry and is also referred to as the internal diameter. In suchliners, the geometry is characterized by features that are concentric toan axis that runs through the center of the internal diameter.

An acetabular prosthesis may be fixed in the reamed acetabulum of apatient. Such a prosthesis may include a cup (or a cup and linerassembly) that is fixed either by placing screws through apertures inthe cup or by securing the cup with cement. In some cases, only a lineris cemented in a patient due to poor bone stock. In other cases, a cuphaving a porous surface may be press fit into the reamed acetabularsurface.

A femoral prosthesis used in total hip arthroplasty generally includes aspherical or near-spherical head attached to an elongate stem with aneck connecting the head and stem. In use, the elongate stem is locatedin the intramedullary canal of the femur and the spherical ornear-spherical head articulates relative to the acetabular component.Femoral prostheses used in total hip arthroplasty procedures may or maynot differ from an endoprosthesis used in a hemi-arthroplasty, describedbelow. However, the femoral head of each type prosthesis is generally astandard size and shape. Various cups, liners, shells, stems and othercomponents may be provided in each type arthroplasty to form modularprostheses to restore function of the hip joint.

Hemi-arthroplasty refers to replacing part of a hip joint, such asreplacing a femoral component so that a femoral prosthesis articulatesagainst natural body tissue in the patient's acetabulum. A femoralprosthesis implanted during a hemi-arthroplasty is generally referred toas an endoprosthesis. Generally, an endoprosthesis includes a stem, ahead, and may include additional components such as shells and liners.Current endoprosthesis designs include (1) monoblock; (2) two-component;(3) three-component; and (4) five-component designs. A monoblockendoprosthesis is a one-piece structure including a femoral stem andhead. Polarity refers to the number of articulating surfaces aprosthesis contains. A monoblock endoprosthesis has one articulationsurface between the head and the patient's natural acetabulum, and istherefore referred to as monopolar.

A two-component endoprosthesis includes a femoral component and a shell.The femoral component may include a modular head and stem. Atwo-component design may be bipolar, so that the head articulatesrelative to the shell and the shell articulates relative to theacetabulum. A three-component endoprosthesis includes a femoralcomponent, a liner, and a shell. Similar to a two-component design, thefemoral component may include a modular head and stem. A three-componentendoprosthesis may either be: bipolar, in which the liner is fixed inthe shell; or tripolar, in which the head articulates relative to theliner, the liner articulates relative to the shell, and the shellarticulates relative to the acetabulum. A five-component endoprosthesisincludes a femoral component (which may include a modular head andstem), a first liner, a first shell, a second liner, and a second shell.Both of the first and second liners are fixed inside each of the firstand second shells. Therefore, this design is a tripolar design: thesecond shell is free to articulate with respect to the acetabulum, themodular head of the femoral component articulates with respect to thefirst liner and the first shell articulates relative to the secondliner. Thus, endoprostheses may be described both with respect to thenumber of components and with respect to the number of articulatingsurfaces as installed in a patient. Some current designs may alsoinclude a mechanical device, such as a snap-ring, for constraining thefemoral head, further described below.

Endoprostheses, as well as total hip prostheses, may also be describedas constrained and non-constrained prostheses. Non-constrainedprostheses rely on the downward force of the body through the joint andthe tension created by the soft tissue, including the muscles, ligamentsand tendons, to retain the prosthesis in its implanted position. Otherprostheses include mechanisms for preventing dislocation of thecomponents, such as the implant stem head. Typically, these prostheseshave restraint mechanisms that result in a smaller range of motion ofthe hip joint, and are generally referred to as “constrained”components.

One example of a restraint mechanism is a shell or liner having greaterthan hemispherical coverage around the head such that the head isconstrained within the internal diameter, thus preventing subluxationand dislocation. In contrast to standard liners, constrained linersemploy an extended, elevated portion over a segment of the periphery ofthe liner internal diameter in order to increase coverage of the femoralhead and thus reduce the likelihood of dislocation and aid in reductionof the head should subluxation occur. While use of a constrainedcomponents is generally not desirable due to resulting decreased rangeof motion, the use of constrained components may be beneficial in casesof tenuous stability in order to avoid dislocation. See e.g. T. Cobb, etal., The Elevated-Rim Acetabular Liner in Total Hip Arthroplasty:Relationship to Postoperative Dislocation, Journal of Bone and JointSurgery, Vol. 78-A, No. 1, January 1996, pp. 80-86. However, constrainedcomponents have a reduction in the arc of motion to contact in thedirection of the elevated lip segment, thus, there is a substantial lossof overall range of motion compared to a standard liner. An implant stemhead constrained by a shell or liner may dislocate if the femoralcomponent rotates beyond the range of motion permitted by the assembly.Dislocation may occur because the edge or lip of the liner or shell thatretains the implant stem head acts as a fulcrum about which the femoralcomponent pivots, thereby causing the implant stem head to dislocatefrom its position within the liner or shell of the prosthesis.Dislocation of a hip prosthesis is painful and often requires medicalintervention.

Three component bipolar endoprostheses including polyethylene liners areknown in the industry, and suffer from at least three major clinicalproblems. First, the vast majority of articulation occurs between theliner and the shell, and it is not uncommon to obtain almost no relativemotion between the shell and the acetabulum. Second, there is often aconsiderable amount of polyethylene wear debris generated from thedevice due to fatigue loading of the liner. Finally, there is a lowerlimit to the size of the shell due to the need to incorporate a standardhead size and an appropriately thick liner. Current solutions to theseproblems include a design having a ceramic shell, a ceramic head and apolyethylene snap ring, which locks the head in the shell. Such designsfrequently lead to polyethylene wear and have a complex assembly.Another solution has been use of a unipolar monoblock device, which doesnot require a liner but which results in excessive wear of theacetabulum.

Thus, there exists a need for a prosthetic component capable ofretaining an implant stem head to prevent it from dislocating whileproviding a larger range of motion than is allowed by conventionalconstrained prostheses. There is also a need for a prosthetic componentcapable of retaining an implant head to prevent it from dislocatingwhile eliminating the requirement of an inner bearing surface, or liner.

SUMMARY OF THE INVENTION

Set forth below is a brief summary of systems and methods according tothe invention that addresses the foregoing problems and providesbenefits and advantages in accordance with the purposes of the presentinvention as embodied and broadly described herein. A prosthesisaccording to one embodiment of this invention provides a constrainedprosthesis with an increased range of motion over current constrainedprostheses. According to one embodiment of this invention, a constrainedprosthesis prevents dislocation of a femoral component. A prosthesis ofthis invention may be used with humans and animals and may be used withconventional hip prostheses including endoprostheses and prosthesis usedin total hip arthroplasty.

A containment system according to one embodiment of this inventionincludes an implant structural member having a generally spherical outersurface and having a cavity and opening adapted to receive an implantstem head or other prosthetic component. The opening of the cavity ofthe implant structural member includes a lip having a web for retainingthe implant stem head within the cavity and preventing it fromdislocating. The web comprises only a portion of the lip forming theopening. As a result, the femoral component is able to travel through alarger range of motion than the range of motion provided a femoralcomponent coupled to a conventional constrained prosthesis. In oneembodiment, a portion of an outer surface of the implant stem head isconfigured to correspond to the web on the shell. An implant stem headaccording to one embodiment of this invention includes an apertureadapted to receive a femoral stem component.

A containment system according to one embodiment of this invention isassembled by first aligning the surface of the implant stem headconfigured to correspond to the web of the implant structural member ina first orientation so that it corresponds to the web. The implant stemhead is then inserted into the implant structural member and rotateduntil the head aperture is visible through the opening in the implantstructural member. The implant stem head cannot be oriented in the firstorientation where the surface of the head corresponds with the web whilethe femoral stem component is coupled to the implant stem head.Therefore, the implant stem head cannot be removed from the implantstructural member unless the femoral stem is first removed. As a result,the femoral component cannot dislocate while positioned within apatient.

One feature of a containment system according to one embodiment of thisinvention is the elimination of the risk of dislocation of a femoralcomponent.

Another feature of a containment system according to one embodiment ofthis invention is a constrained prosthetic component providing anincreased range of motion.

Yet another feature of a containment system according to one embodimentof this invention is that a containment system is inexpensive tomanufacture and includes few parts.

Another feature of a containment system according to one embodiment ofthis invention is the elimination of the need for a polyethylene liner,allowing the use of a larger head diameter with a resulting increase inrange of motion and elimination of the possibility of polyethylene wear.

Yet another feature of a containment system according to one embodimentof this invention is a containment system that is easy to assemble andthat requires no additional instruments for assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate preferred embodiments of the presentinvention and, together with the description, disclose the principles ofthe invention.

FIG. 1 is a perspective view in cross-section of a containment systemaccording to one embodiment of this invention incorporated within atwo-component endoprosthesis.

FIG. 2 is an exploded perspective view of the system of FIG. 1.

FIG. 3 is an exploded perspective view of an implant structural memberand implant stem head of one embodiment of a containment systemaccording to this invention.

FIG. 4 is an exploded perspective view of the shell and implant stemhead of FIG. 3, rotated 180 degrees.

FIG. 5 is a top plan view of the implant structural member of FIG. 1.

FIG. 6 is a top plan view of an implant structural member according toan alternative embodiment of this invention.

FIG. 7 is a side elevation view in cross-section of the implantstructural member and implant stem head of FIG. 3.

FIG. 8 is a side elevation view in cross-section of the implantstructural member and implant stem head of FIG. 7, as assembled.

FIG. 9 is a side elevation view in cross-section of the implant stemhead and shell of FIG. 8 after the head has been rotated to receive astem.

FIG. 10 is a side elevation view in cross-section of the implant stemhead, shell and stem of FIG. 1, as assembled.

FIG. 11 is a side elevation view in cross-section of the implant stemhead, shell and stem of FIG. 1, as assembled and rotated.

FIG. 12 is a perspective view in cross-section of a containment systemaccording to one embodiment of this invention incorporated within athree-component endoprosthesis.

FIG. 13 is a perspective view in cross-section of a containment systemaccording to one embodiment of this invention incorporated within afive-component endoprosthesis.

FIG. 14 is an exploded perspective view of a containment systemaccording to an alternative embodiment of this invention.

FIG. 15 is a side elevation view in cross-section of the shell andimplant stem head of FIG. 14.

FIG. 16 is a side elevation view in cross-section of the shell andimplant stem head of FIG. 14, as assembled.

FIG. 17 is a side elevation view in cross-section of the shell andimplant stem head of FIG. 14, as assembled and rotated.

DETAILED DESCRIPTION OF THE DRAWINGS

Methods, systems and devices according to this invention seek to providea containment system for preventing prosthetic articulating surfacesfrom separating from each other while allowing an increased range ofmotion over conventional constrained prosthesis. One embodiment of acontainment system according to this invention seeks to provide a hipprosthetic component for constraining an implant stem head of a femoralcomponent while providing an increased range of motion. Generally, acontainment system according to one embodiment of this invention,includes an implant structural member having a cavity and an opening toreceive an implant stem head. The opening includes a lip and a web alonga portion of the lip. The containment system also includes an implantstem head having a cooperating surface with a shape that correspondswith the shape of the web. The implant stem head is adapted so that itmay only be inserted into and removed from the implant structural memberwhen the cooperating surface of the head is aligned with the web.

A containment system according to one embodiment of this invention isassembly by aligning the cooperating surface of the implant stem headwith the web of the implant structural member and inserting the implantstem head into the implant structural member. The implant stem head isthen rotated and the femoral stem component is attached. The implantstem head can only be removed from the implant structural member if thestem is first removed from the head, allowing the cooperating surface ofthe implant stem head to be oriented with the web. In this manner, thehead is constrained in the implant structural member as long as the stemand head are assembled.

Consider one example of systems and devices according to this invention.Containment system 10 is illustrated as a two-component bipolarendoprosthesis in FIGS. 1 and 2. This containment system 10 includes animplant structural member 14, such as a shell, adapted to receiveimplant stem head 12, which is adapted to be coupled to femoral stemcomponent 13, the implant stem head and femoral stem component forming afemoral component when assembled. As shown in FIGS. 1 and 2, implantstructural member 14 is adapted to fit within acetabulum 11 and isadapted to capture and retain implant stem head 12. Implant structuralmember 14 includes a cavity 16 that has a generally spherical shape andthat is formed by an inner surface 18 and an opening 20. Opening 20includes lip 36 having web 22, which reduces the size of opening 20 sothat opening 20 forms a D-shape, as shown in FIG. 5. In an alternativeembodiment, shown in FIG. 6, web 22 includes curved edge 31, whichallows the femoral stem component to rotate through a slightly largerrange of motion. In other embodiments, web 22 has other suitable shapes.

Implant stem head 12 may or may not extend beyond opening 20 of implantstructural member 14. For example, as shown in FIGS. 1-2 and 10-11,implant stem head 12 extends beyond opening 20 of implant structuralmember 14, but head 12 and member 14 may also be adapted so that head 12does not extend beyond the opening 20. For example, as shown in FIGS.3-4 and 7-9, implant structural member 14 is adapted to receive implantstem head 12 such that head 12 does not extend beyond opening 20 ofimplant structural member 14, but may also be adapted so that head 12does extend beyond the opening 20. In one embodiment, the plane of web22 is at a right angle to the lip 36; however, in an alternativeembodiment, the plane of web 22 is at any suitable angle relative to thelip. The description of the two component containment system 10 isgenerally applicable to all embodiments shown in FIGS. 1-11, with theonly difference being the positioning of the implant stem head relativeto the implant structural member, as described above.

As shown in FIG. 7, first tangent line 30 and second tangent line 32 arenot parallel. Instead, first tangent line 30 is at an angle α that isless than 90 degrees relative to reference line 34, which is parallel tosecond tangent line 32. Web 22 is formed as a continuation of innersurface 18 and maintains the same radius of curvature as other portionsof inner surface 18. Thus, web 22 is formed by extending inner surface18 beyond reference line 34 to form angle α, as shown in FIG. 7. In thisconfiguration, web 22 decreases the diameter across opening 20 and doesnot create a surface likely to cause unnecessary wear on implant stemhead 12.

Implant stem head 12 is generally spherical in shape and includes acooperating surface 24 that enables implant stem head 12 to be insertedwithin cavity 16 of implant structural member 14. Surface 24 may beflat, as shown in FIGS. 1-4, or may be formed of alternative shapes thatcorrespond to the shape of the web. As shown in FIG. 7, cooperatingsurface 24 is positioned on implant stem head 12 so that a diameter 15of implant stem head 12 taken perpendicular to cooperating surface 24and traveling through the center of implant stem head 12 is slightlysmaller than distance 17 within opening 20. This configuration allowsimplant stem head 12 to be inserted into cavity 16 by aligningcooperating surface 24 with web 22. Orienting cooperating surface 24 inthis manner allows implant stem head 12 to be inserted into cavity 16past web 22, as shown in FIG. 8.

After implant stem head 12 has been completely inserted into cavity 16of implant structural member 14, head 12 is rotated so that head 12 isretained in cavity 16. Specifically, after implant stem head 12 isrotated, head 12 cannot be removed from implant structural member 14because the diameter 19 of implant stem head 12, shown in FIG. 7, isgreater than distance 17 of opening 20, and cooperating surface 24 is nolonger oriented to correspond with web 22, as shown in FIG. 9. A femoralstem component 13 is then coupled to head 12, preventing alignment ofcooperating surface 24 with web 22 so that head 12 is retained inimplant structural member 14. Once femoral stem component 13 has beencoupled with head 12, head 12 cannot be removed from shell 14 untilfemoral stem component 13 has been removed because stem 13 prevents head12 from being rotated so that cooperating surface 24 aligns with web 22.

As shown in FIGS. 7-9, implant stem head 12 includes cavity 26, which isadapted to receive femoral stem component 13 and which is generallycylindrical. In an alternative embodiment, cavity 26 may be conical,cubical, or any other suitable shape. In other alternative embodiments,cavity 26 may include threads, barbs, rings or any other suitablemechanical connectors to couple head 12 to stem 13. In yet anotherembodiment, adhesive or cement may be used to couple head 12 to femoralstem component 13.

In one embodiment, opening 28 of cavity 26 of implant stem head 12 ispositioned on cooperating surface 24. In this embodiment, opening 28 ispositioned to receive stem 13 only after implant stem head 12 has beenrotated to expose opening 28, as shown in FIG. 9. Attachment of stem 13to implant stem head 12 prevents implant stem head 12 from being rotatedso that implant stem head 12 can be removed from cavity 16, as shown inFIG. 10. Specifically, after implant stem head 12 has been attached tostem 13, stem 13 contacts the lip 36 of opening 20 and prohibits implantstem head 12 from being rotated any further, as shown in FIG. 11. Thus,lip 36 of opening 20 defines the range of motion of femoral stemcomponent 13. In another embodiment, opening 28 is positioned withinhead 12 at a location other than within surface 24. For instance,opening 28 may be located adjacent to surface 24 or in another locationon head 12. During use, web 22 is preferably positioned superiorly, asshown in FIG. 2, in order to maximize range of motion and minimize thepossibility of subluxation.

In one embodiment according to this invention, off-axis eccentricity isprovided. In one embodiment, negative eccentricity is provided, forcingrotation of the implant structural member relative to the acetabulum. Inanother embodiment, positive eccentricity is provided, resulting in anincreased range of motion of the femoral component. For example, asshown in FIGS. 7-9, the center of rotation 37 of implant stem head 12 isnot positioned in the same location as the center of shell 39. Center ofrotation 37 of implant stem head 12 is positioned closer to opening 20than the center of shell 39. As a result, the femoral component is lessrestricted than if both center of rotation 37 and the center of shell 39were located in the same position.

In one embodiment of this invention, implant stem head 12 is ceramic andimplant structural member 14 is a ceramic shell. This embodiment doesnot include polyethylene in any form, reducing wear debris. In oneembodiment, implant stem head 12 is larger than a conventional head,allowing an increased range of motion resulting from the increaseddiameter of the assembly. In one embodiment, the throat diameter of theshell is reduced to slightly less than a complete hemisphere, furtherincreasing range of motion.

An alternative embodiment of a containment system according to thisinvention is illustrated as a three-component tripolar endoprosthesis inFIG. 12. Containment system 41 is very similar to the two-componentembodiment described above, with the retention mechanism located withinliner 38. For example, containment system 41 includes a shell 46 havinga cavity 40 for receiving liner 38. Shell 46 is adapted to be receivedin acetabulum 43. Liner 38 is adapted to be received in cavity 40 ofshell 46 and includes a cavity 47 adapted to receive implant stem head48. Cavity 47 of liner 38 is defined by an inner surface 42 and opening44. Inner surface 42 can be formed from any conventional process. Asdescribed in the two-component endoprosthesis embodiment above, a web 50on the lip of liner 38 is positioned to capture an implant stem head 48and to prevent dislocation of implant stem head 48 while the stem 45 iscoupled to head 48.

Yet another alternative embodiment of a containment system according tothis invention is illustrated as a five-component endoprosthesis in FIG.13. Similar to the embodiment of FIG. 12, the retention mechanism ofcontainment system 72 is located on first liner 74, which is adapted toreceive implant stem head 76. First shell 78 is adapted to receive firstliner 74, while second liner 80 is adapted to receive first shell 78.Second liner 80 is received in second shell 82, which is received inacetabulum 84. First liner 74 includes a web 86 and captures and retainshead 76 as described in the embodiments above.

In an alternative embodiment, shown in FIGS. 14-17, a containment systemincludes shell 54 having two webs 56, 57, each positioned on opposingsides of opening 58 of shell 54. Implant stem head 60 includes twocooperating surfaces 62, 63 that correspond with webs 56, 57 so thatimplant stem head 60 may be inserted into cavity 64 of shell 54. In thisembodiment, the containment system functions in the same manner as theembodiments having a single web. For example, implant stem head 60 ispositioned so that surfaces 62, 63 correspond with webs 56, 57 as shownin FIGS. 14 and 15. Implant stem head 60 is then inserted into shell 54,as shown in FIG. 16. Implant stem head 60 is then rotated until cavity66 of head 60 is capable of receiving a femoral stem component 70through opening 68, as shown in FIG. 17. Other alternative embodimentsof a containment system according to this invention may include shellsor liners having any appropriate number of webs that may be configuredas shown or in alternative configurations, such as three, four or fivewebs and having heads corresponding to the webs.

The components of the various prostheses described may be made frommetal, such as stainless steel and titanium, ceramic, plastic, such aspolyethylene, or any other suitable material. In one embodiment, all ofthe components of a two-component bipolar endoprosthesis are metal. Inan alternative embodiment, all of the components of a two-componentbipolar endoprosthesis are ceramic. In another embodiment, the shell andimplant stem head of a three-component endoprosthesis are either metalor ceramic, while the liner is polyethylene.

The embodiments of a containment system as described above all relate toendoprostheses used in hemi-arthroplasty. Similar embodiments of acontainment system according to this invention may be incorporated intoa prosthesis used in total hip arthroplasty, using a web to constrain afemoral head component. Alternative embodiments of a containment systemaccording to this invention may be utilized to capture and retain otherprosthetic components. For example, a web may be incorporated into asecond liner of a five component endoprosthesis in order to capture andretain the first shell and first liner assembly. Additionally,embodiments of a containment system according to this invention may alsobe used to capture components of other orthopedic prosthesis, such asshoulder prosthesis and small finger joint prosthesis. For example, aconstrained shoulder prosthesis may include a web to capture and retainthe humeral head.

One method of using one form of structure according to this invention isas follows. Implant stem head 12, shown in FIG. 7, is positioned to beinserted into shell 14 by orienting implant stem head 12 in a firstorientation so that cooperating surface 24 corresponds with web 22. Asshown in FIG. 8, implant stem head 12 is inserted into shell 14 so thatthe outside surface of implant stem head 12 contacts inside surface 18of shell 14. Implant stem head 12 is then placed in a second orientationby rotating head 12 so that cavity 26 is exposed and in position toreceive femoral stem component 13. After femoral stem component 13 hasbeen coupled to implant stem head 12, head 12 cannot be removed fromshell 14 unless stem 13 is first removed because stem 13 prevents head12 from being oriented in a first orientation where cooperating surface24 corresponds with web 22. The endoprosthesis is then surgicallyimplanted, so that the stem of the femoral component is implanted intothe femur of a patient. The endoprosthesis is then implanted in theacetabulum of a pelvis.

The foregoing is provided for purposes of illustrating, explaining, anddescribing embodiments of this invention. Modifications and adaptationsto these embodiments will be apparent to those skilled in the art andmay be made without departing from the scope or spirit of this inventionor the following claims.

1. A containment system comprising: (a) a shell, the shell comprising acavity and an outside surface and including at least one opening on theoutside surface providing access to the cavity; (b) a liner, the linercomprising: an outer surface; a lip; and an inner surface, the innersurface forming a cavity and including a web, wherein the web extendsaround a portion of the lip to form a D-shaped opening; (c) an implantstem head, the implant stem head comprising: a generally spherical bodyhaving a surface configured to correspond to the web enabling theimplant stem head to be inserted into the liner when the implant stemhead is in a first orientation; and a cavity; and (d) a separate femoralstem component which may be inserted into the cavity of the implant stemhead, wherein the shell may be received by an acetabulum, and whereinthe liner may be received in the cavity of the shell, and wherein theweb allows the implant stem head to be inserted into the cavity of theliner when the implant stem head is oriented in the first orientationand constrains the implant stem head within the cavity of the liner whenthe implant stem head is oriented in a second orientation such that theimplant stem head may articulate within the liner but cannot be removedfrom the liner once it is attached to the stem.
 2. A containment systemcomprising: (a) a shell, the shell having a generally hemisphericalshape and comprising a cavity and an outside surface and including atleast one opening on the outside surface providing access to the cavity,wherein the shell is received by an acetabulum; (b) a liner, the linerhaving a generally hemispherical shape and comprising: an outer surface;a lip; and an inner surface, the inner surface forming a cavity andincluding a web, wherein the web extends around a portion of the lip toform a substantially D-shaped opening, and wherein the liner is receivedin the cavity of the shell and may articulate within the cavity of theshell; (c) an implant stem head, the implant stem head comprising: agenerally spherical body having a substantially planar surfaceconfigured to correspond to the web enabling the implant stem head to beinserted into the liner when the implant stem head is in a firstorientation; and a cavity; and (d) a separate femoral stem componentwhich may be inserted into the cavity of the implant stem head, whereinthe inner surface of the liner is shaped to correspond generally to theouter surface of the implant stem head, and wherein the web is shaped tocooperate with the implant stem head as the implant stem headarticulates relative to the liner, and wherein the web allows theimplant stem head to be inserted into the cavity of the liner when theimplant stem head is oriented in the first orientation and constrainsthe implant stem head within the cavity of the liner when the implantstem head is oriented in a second orientation such that the implant stemhead may articulate within the liner but cannot be removed from theliner.
 3. A containment system comprising: (a) an implant structuralmember, the implant structural member comprising: an outer surface; alip; and an inner surface, the inner surface forming a cavity andincluding a web, wherein the web extends around a portion of the lip toform a substantially D-shaped opening; (b) an implant stem head, theimplant stem head comprising: a generally spherical body having asubstantially planar surface configured to correspond to the webenabling the implant stem head to be inserted into the implantstructural member when the implant stem head is in the firstorientation; and a cavity; and (c) a separate femoral stem componentwhich may be inserted into the cavity of the implant stem head, whereinthe outer surface of the implant structural member may be received by ashell, and wherein the implant stem head may be received by the implantstructural member, and wherein the web allows the implant stem head tobe inserted into the cavity of the implant structural member when theimplant stem head is oriented in a first orientation and constrains theimplant stem head within the cavity when the implant stem head isoriented in a second orientation.